This invention relates to a method for reducing emissions of organic pollutants while roasting metal-containing ores and concentrates, particularly metal sulfides, in the presence of air.
The oxidation roasting of metal sulfide ores and concentrates, to remove sulfur and render the metal more amenable to further chemical or metallurgical purification and conversion, has been widely used for many years. Such a process can be efficiently conducted in multiple-hearth furnaces, of which several variations are known, including those referred to as Herreshoff, Nichols, Nichols-Herreshoff, Wedge, Skinner, Pacific and others.
Chemical reactions occurring within the furnace are primarily those of oxidation, having the net effect of converting the metal sulfide into metal oxides and gaseous oxides of sulfur. Such conversion is accompanied by the evolution of considerable heat energy, which must be dissipated rather quickly to avoid prolonged exposure of the formed metal oxides at such temperatures as could cause their vaporization, melting or fusion with other materials in the furnace. In addition, heat removal is necessary to protect the interior components of the furnace, since exposure to higher temperatures can markedly reduce the service life of many commonly used construction materials.
In the past, this heat dissipation has been effected by drawing air through the furnace, in amounts far exceeding that necessary to supply the oxygen required by the stoichiometry of the desired combustion reactions. Furnace temperature levels are thus controlled by increasing or decreasing the quantity of cooling air which flows through the furnace.
Unfortunately, however, the introduction of large excesses of air has resulted in dilution of the sulfur oxide concentration in waste gases exiting the furnace to a level considerably below that which would be obtained with an air flow close to the stoichiometric air required for combustion. Since the release to the atmosphere of large quantities of sulfur oxides has become environmentally unacceptable, it has been necessary to provide sulfur removal treatment for large volumes of waste gases which contain low concentrations of sulfur oxides. Such treatment is technologically more difficult and requires much greater capital expenditures than would be needed for smaller volumes of gases.
In addition to the sulfur oxide pollution problems, quantities of organic vapors must be removed from gases exiting the furnace. These organics are primarily residues from flotation concentration techniques performed on the sulfide ores prior to the roasting process, and are volatilized during heating to the uppermost hearths of a furnace, prior to initiation of the oxidation reactions. The organic vapors are diluted in the same manner as are the sulfur oxides, and are also very difficult to remove when large volumes of cooling air are used in a furnace.
A further disadvantage resulting from the necessity for excess air is the increased instability in furnace operating conditions. Changes in ambient air temperature (e.g., from daytime to evening, or summer to winter), humidity, etc. can cause significant alterations in temperature levels within the roaster, which are made more severe with increased air volumes. Because of this, the level of operator attention and control adjustments needed is significantly increased.
One technique for decreasing the volume of excess air required is that of Grimes in U.S. Pat. No. 4,034,969, wherein a system for spraying cooling water onto individual hearths is described. Advantages claimed for this system are, inter alia, closer control of temperature on each hearth and facilitation of air pollution control.
Fitton, in British Pat. No. 1,366,712, has described a recycle system for waste gases in a furnace for roasting pelletized lead or zinc sulfides, in which a reduction in the amount of supplied air is contemplated. The resulting increase in sulfur dioxide concentration of the recycled gases is claimed to control the rate at which the feed pellets oxidize.
An article in the Bulgarian language by Paleshnikov et al., abstracted in Chemical Abstracts, Vol. 80, 7298u (1974), reports an increased sulfur dioxide content from about 2 percent to about 5.5 percent, due to partial recirculation of gases during the roasting of lead ore concentrates. However, the type of roaster used is not given in the abstract.
Accordingly, it is an object of this invention to provide a method which will decrease the volume of supplied air in a multiple-hearth furnace for roasting metal sulfides, thereby increasing sulfur oxide concentration and reducing the capital costs and operating expenses of air pollution abatement.
It is a further object to capture a significant portion of the organic vapors in an ore roasting process, and utilize those vapors to supply some part of the fuel requirements of the furnace.
A still further object is to stabilize furnace operating conditions through the reduction in volume of supplied air.
Another object is to lessen the heat losses in a multiple-hearth furnace by recycling hot exhaust gases back into cooler portions of the furnace.
These, and other objects, will more clearly appear from consideration of the following disclosure and accompanying drawings.